An electromagnetic steel sheet is used as a soft magnetic part in an electric device having an electromagnetic valve, a motor, or a power source circuit. Magnetic characteristics that a large flux density can be obtained by applying a small magnetic field and that can respond sensitively to a change in the magnetic field from outside are desired in the soft magnetic part.
In the case of using such a soft magnetic part in an AC magnetic field, an energy loss called an iron loss occurs. This iron loss can be represented as a sum of a hysteresis loss and an eddy current loss. The hysteresis loss is equivalent to the energy that is necessary to change the flux density of the soft magnetic part. Because the hysteresis loss is proportional to the operating frequency, it becomes dominant mainly in a low frequency range of 1 kHz or less. Further, the eddy current loss referred to herein is an energy loss that is generated by an eddy current flowing mainly in the soft magnetic part. Because the eddy current loss is proportional to the square of the operating frequency, it becomes dominant mainly in a high frequency range of 1 kHz or more.
A magnetic characteristic that reduces the generation of this iron loss is desired for the soft magnetic part. In order to realize this, it is necessary to make the magnetic permeability μ, the saturated flux density Bs, and the electric resistivity ρ large, and the coercive force Hc of the soft magnetic part small.
Because of the advancements in making the operating frequency higher towards manufacture of high output and high efficiency devices in recent years, a dust core that has smaller eddy current loss compared with the electromagnetic steel sheet has been attracting attention. This dust core is made of a plurality of composite magnetic particles, and a composite magnetic particle includes a metal magnetic particle and an insulating film coating its surface.
In order to lower the hysteresis loss among the iron loss of the dust core, the coercive force Hc of the dust core may be made small by removing distortion and dislocation in the metal magnetic particles and making movement of a magnetic wall easy. In order to sufficiently remove the distortion and the dislocation in the metal magnetic particles, it is necessary to perform a heat treatment on the molded dust core at a high temperature of 400° C. or more, preferably a high temperature of 550° C. or more, and more preferably a high temperature of 650° C. or more.
However, the insulating film is made of an iron phosphate non-crystalline compound having high adhesiveness to powders that are obtained by a phosphating treatment or the like, and rich in elasticity for the reason that a following property toward powder deformation is desired at molding, and sufficient high temperature stability is not obtained. That is, when the heat treatment is performed on the dust core at a high temperature of 400° C. or more for example, the insulating property is spoiled because constituting metal elements in the metal magnetic particles diffuse and invade into the non-crystalline part, for example. Therefore, when it is intended to lower the hysteresis loss by the high temperature heat treatment, the electric resistivity ρ of the dust core decreases, and there has been a problem that the eddy current loss becomes large. Making an electric device small and efficient, and providing the device with large output has been required in recent years, and in order to satisfy these requirements, it is necessary to use an electric device in a higher frequency range. If the eddy current loss becomes large in the high frequency range, it becomes a hindrance to make the electric device small and efficient, and providing the device with large output.
Therefore, a technique that can improve the high temperature stability of the insulating film is disclosed in Japanese Patent Laying-Open No. 2003-272911 (Patent Document 1) for example. In the above-described Patent Document 1, a soft magnetic material is disclosed made of composite magnetic particles having an aluminum phosphate insulating film with high temperature stability. In the above-described Patent Document 1, the soft magnetic material is manufactured by the following method. First, an insulating coating solution containing phosphate including aluminum and heavy chromate including potassium is jetted onto iron powder. Next, the iron powder jetted with the insulating coating solution is maintained at 300° C. for 30 minutes, and then 100° C. for 60 minutes. With this operation, the insulating film formed on the iron powder is dried. Next, the iron powder on which the insulating film is formed is pressure-molded, the heat treatment is performed after the pressure-molding, and a soft magnetic material is completed.    Patent Document 1: Japanese Patent Laid-Open No. 2003-272911